Various types of liquid coating compositions have been applied to metallic substrates and baked thereon in order to protect the substrates against corrosion. Certain of such coatings are applied in conventional metal coil coating processes, and they must be sufficiently adherent and flexible to resist cracking, chipping and peeling. Their attributes and their shortcomings are discussed in U.S. Pat. No. 5,001,173 which has an assignee in common with this application and which is incorporated herein by reference.
An aqueous barrier coating composition containing an epoxy resin and chromium trioxide is disclosed in U.S. Pat. No. 5,001,173 and one containing polytetrafluoroethylene as a permanent internal lubricant is disclosed in copending patent application Ser. No. 08/237,953 filed on May 3, 1994, now abandoned which also has an assignee in common with the instant application and which also is incorporated herein by reference.
Some of the most popular corrosion resistant coatings currently in commercial use also have internal lubricants but must be washed off before primer and finish paints are applied electrostatically; corrosion resistance is thus lost and nothing is done for paint adhesion. The loss of corrosion resistance means that sheets and parts must be removed from an E-coat line whenever the line is shutdown for cleaning and maintenance. Zinc residues from some prior art corrosion preventing compositions gum up the washing and disposal equipment. Excellent die lubrication is afforded to metal stamping operations by the internally lubricated coating of the aforementioned '953 application but the coating weight necessary for such lubrication causes the organic barrier to also act as an insulator, reducing the surface conductivity of the coated substrate.
Another problem that must be solved when metal substrates are to be protected from corrosion is the growing demand for weldable coated sheets. Due to the physical limitations of inhibitive pigment concentration and the thickness of the films provided, it is difficult to provide both adequate corrosion resistance and weldability. Often, the choice is between the use of a higher current to produce the weld or decreasing the thickness of the protective film.
Primers and finish coating compositions are known to contain electrically conductive pigments so as to facilitate the electrostatic spraying of substrates coated therewith. The conductive pigments and coating compositions containing them are described in Japanese Unexamined Patent Publication 62-181371, and U.S. Pat. Nos 4,655,966 and 5,114,756; the U.S. patents are incorporated herein by reference. None of these publications describe a coating composition which contains chromic acid as a corrosion preventing additive.